View Compilation Error

0_0_24474715_12802.cpp: In function 'vi next()':
0_0_24474715_12802.cpp:34:13: error: 'expr' was not declared in this scope
 vi v = expr(); p ++;
             ^
0_0_24474715_12802.cpp: In function 'int main()':
0_0_24474715_12802.cpp:67:13: error: 'expr' was not declared in this scope
 vi v = expr();
             ^